In the recent past, automotive engine designers have focused their efforts on engine down-sizing activities. The goal of the automotive engine engineer has been to increase engine performance of smaller displacement engines to allow them to be competitive with their larger displacement (heavier) counterparts. This has been done for the purpose of enhancing fuel economy by reducing overall vehicle weight. As a result, the engine performance of engines has risen to the extent that the engine block has become a more highly-stressed component, and the subject of much debate. This is particularly true of the present class of highly efficient European passenger car turbo-diesel engines. These engines have attained peak cylinder pressures approaching 200 bar, which has stressed the engine block far beyond historical levels. See Prior art FIG. 1.
For years the only cost effective material of choice for automotive engine block construction was gray cast iron. This material was used for its ability to be designed with for “infinite life” due to the nature of iron-based crystalline structures (body centered cubic crystalline structures). The material provided a good tradeoff for initial cost and machine-ability. The cost of automotive engine blocks was reasonable, and the engine could be easily re-built by renewing the engine and/or crankshaft bearing bores, and installing an over-sized piston.
As weight became more of an issue for automobile designers, we began to see more aluminum block concepts find their way into production. The trend began as a way to save weight in performance applications, but since has been used in all types of vehicles to reduce weight and hence rolling friction, and provide superior fuel economy.
Any piston engine is simply a collection of pressure vessels that utilizes a crank rocker (crankshaft) mechanism to impart the expansion work of gases for the purpose of delivering useful work. See Prior art FIG. 4. The challenge to engine designers has always been to develop an elegant structure that uses no more material than necessary to deliver reliable power. With recent advances in diesel technology, the necessity to optimize engine block and crank shaft design has become evident. Modem diesel engine combustion creates peak gas forces in the region of 200 bars peak pressure. (See Prior Art FIG. 3) This is more than twice the pressure of a typical gasoline automotive engine, and 3-4 times that seen in aircraft engines. The two most massive engine components by weight have traditionally been the engine block and crankshaft assembly.
Although it is well known by engineers that modem diesel engines are more thermally efficient, the challenge for weight-sensitive applications has been to integrate diesels into a compact weight-efficient package. Nowhere is this more critical than in the design of aero applications. This application demands that an engine be lightweight, durable, efficient and powerful. To achieve these characteristics simultaneously, the engineer must go through a thorough a “sizing” study to determine how much engine capacity is sufficient to do the job properly.
Brake Mean Effective Pressure (BMEP), or P in Equation 1, is used to compare the performance of various engine configurations. It is the average pressure over the cycle time that an engine would achieve if it were operating as a constant pressure device.
The basic equation for engine power can be simplified to the following form:Power=PLAN
Equation 1 Definition of Power as a Function of BMEP, Engine Geometry, and Speed Where:    P=Average Pressure on the Piston    L=Stroke Length    A=Piston Area    N=Firing Pulses per MinuteIt should be evident then that given the same power target, the options are limited for the engine designer. It should also be evident that the only way to increase power output of a four-stroke engine is to:    1. Increase capacity; (engine displacement by increasing a combination of L & A)    2. Increase engine speed; (firing pulses per unit time)    3. Increase P; (the average pressure over the cycle)Since the goal is to obtain more specific power, the task of the engine designer is to increase power without a corresponding increase in weight. The significance of this is that by definition, an increase in engine volume will result in an increase in weight. This effectively eliminates option “1” above.
To increase engine speed would certainly result in an increase in specific power. However, this is generally contradictory to engine durability. Things like bearing loading, piston speed, and dynamic vibrations are generally increased with engine speed. A gear reduction can be used to provide torque multiplication when the torque capacity of an engine is insufficient. This is not without penalty, as the design must consider the tradeoff between engine displacement, and gear reduction weight. Another consideration is the gear efficiency (sound characteristic) and torsional behavior of such a gear reduction.
An additional element to consider with regard to increasing engine speed is hat the dimensional accuracy of the engine machined components must be increased to ensure proper dynamic engine behavior. This fact translates directly to increased manufacturing costs which certainly must be taken into consideration in the construction of a light-weight, high speed engine.
So, the last parameter that is increased in the power equation becomes the mean pressure over the cycle duration. In gasoline and diesel engines the use of supercharging has achieved this effect. The peak cylinder pressure has also been practically raised until the limitations on engine block materials have been pushed to their physical limits. In extreme cases, cast iron cylinder heads, or steel inserted heads are being put into production to meet the demands of these high pressure diesel engines.
High pressure simply translates to high component stress in many aspects of the design. The higher stress means that we have to more carefully pursue the effective use of materials to ensure an efficient design.
The limitations of an engine design are typically those imposed by the selected materials of construction. The properties of any given material are readily tested in the usual methods such as the tension test to identify a materials' strength or the rotating beam which is utilized to test the resistance of a material to fatigue, endurance limit, over many cycles.
The most basic difference is the lack of an endurance limit for aluminum materials and their alloys. (Incidentally, practical experience of the applicants (confirmed by written literature and in foundry discussions) limits peak low-silicon aluminum block stress to values less than 200 N/mm2 and hyper-eutectic aluminum peak stress to under 50 N/mm2, to prevent fatigue cracking over the design life of an automotive engine.) See Prior art FIG. 5. In general, all materials are more sensitive to torsional loading than pure tension loading, and are the least sensitive to compressive stress.
Virtually all material properties degrade with temperature. (See Prior Art FIG. 6) This degradation of material properties is why engine designers' strive to optimize engine cooling systems to ensure that engine durability is achieved in the highest temperature service. It has long been known that the dimensions of an engine can be distorted due to the loads imposed on an engine. This distortion is the source for much of the friction and additional loads present in a running engine in service. In fact, at the highest level of motor-sport competition, engine bores are machined with the engine pre-heated and pre-stressed to the running conditions in service.
In a more conventional sense, the thermal growth can be controlled by the appropriate selection of materials. For example, it is usual practice to select steel and cast iron for crankshaft and engine block materials since they have similar values of thermal expansion. By “matching” materials the engine engineer can assure that sensitive bearing clearances will be maintained at both elevated and reduced temperatures. This allows the bearings to maintain consistent clearance, and perform to their optimal design.
Referring to Prior Art FIG. 7, it is well known that aluminum engines lose clamping tension at low temperatures, and reduce main bearing clearance from the differential shrinkage of the aluminum block and crankshaft. This is detrimental to the life of the engine bearing shells, since most engine damage is likely to occur in cold starts.
Conversely, in “hot running conditions” the bearing clearance in an aluminum block can be so large as to lose the stability of the oil film by excessive side-leakage which can also cause engine bearing damage.
Recently, the aluminum block has begun to surface in passenger car diesels that are a large part of the European market. With fuel economy being a primary focus of this application, weight has become an important factor in the decision matrix. When the duty cycle of a passenger car is considered, the durability of the car engine is not a primary driver for the design. Most automotive applications must endure 500 hours of durability testing, or less depending on the severity of the test cycle, and are not traditionally rebuilt at the end of their service life. In fact, there has been some recent discoveries that the most highly-loaded fastener threads (i.e. cylinder head, main bearing caps) have fatigued to the point that the engine is not serviceable. (See Prior Art FIG. 8).
The decision to select a material need not be an exclusive one. The concept of reinforcement has been used for centuries in concrete construction etc. In fact, the selection of various materials is demonstrated in several production engine blocks. For example, whenever aluminum is used for its high strength/weight ratio as the primary structure of an engine, a secondary treatment such as Mahle's Nikasil® is used in the high wear cylinder bore area. In this way, we have a truly composite structure comprised of aluminum for the frame of the engine, and Nikasil® as the micro-thin bore. This is depicted in Prior Art FIG. 9.
When weight is of primary concern, new materials of construction have recently surfaced. For example, BMW has led the way with the utilization of magnesium as a block structure, and hypereutectic aluminum as the running surface of the cylinder bores.
Hyper-eutectic aluminum is a material that can have silicon content as high as 19%, which allows pistons to run directly on the bore surface without a hardening treatment of the bores. Plasma spaying has also been experimented with in conjunction with chemical and laser etching to achieve a proper surface for oil film formation. See Prior Art FIG. 10.
Other concepts are using mechanically reinforced aluminum engine structures, such as BMW's steel-reinforced aluminum block shown in Prior Art FIG. 11. This system achieves reinforcement through bolt-on steel sections, and optimization of bearing cap features.
Audi and Ford have recently used enhancement of traditional materials like cast irons to minimize weight while retaining the durability characteristics expected of a modem automotive engine. One such material is compacted graphite iron, which is a specialized form of cast iron pioneered by the Sintercast™ company of Sweden.
Audi has made the decision to go with a “modified” case iron known as compacted graphite iron or CGI. The cast iron utilizes proprietary techniques to alter the material on a molecular level to ensure optimum strength in thin sections. The benefits of this material have focused the engineering effort at Audi on the full “optimization” of the engine block, making use of the minimum material necessary to achieve strength, retain bore roundness, and reduce the emitted sound from the structure. This block is depicted in Prior Art FIG. 12.